1.but can be an indefinite adjective with the meaning of some, i.e. a part of something bigger.

Here it is possible to use “quelques” to translate ‘some’ but the meaning is slightly different as quelques means some of or a few of when you say certaines personnes n’aiment pas la plage you mean that it is true, the fact that there are some people who don’t like beach

So certain is more general. (See the drawing in the video)

More Examples:

For some people: pour certaines personnes

Sometimes: certaines fois (parfois, quelques fois)

Some days: certains jours

b. we use the plural and sometimes the singular with a more subtle meaning:

Une certaine personne m’a dit: here you know the person but you don’t want to tell his or her name (the person can be with you or not)

Une personne d’un certain âge: the person is not so young but being polite you don’t want to tell.